Lactic acid fermentation has been gaining increased attention in the recent years primarily due to its importance as a building block in the manufacture of biodegradable plastics. Lactic acid can be produced from various substrates such as whey permeate, starch hydrolysates which are sources of lactose and glucose respectively.
Lately, the potential of lactic acid as a starting feed stock for a new class of renewable biodegradable lactide polymers has been recognized. These biodegradable polymers are considered as a replacement for present plastic materials or for various new uses such as in development of support and attachment membrane used in bone surgery, service plastic ware and containers, medical garments, disposable diapers, yard waste bags etc., in which biodegradability is preferred.
For the different applications, varying purity of grades or lactic acid is used. For technical purposes like in metal and leather, low technical grade lactic acid is used; slightly more purified food grade Lactic acid is used in food related applications. High purity grade is used in pharmaceutical applications. Whereas, in case of use of lactic acid as starting feed stock for lactide polymers, exceptionally pure, thermally stable grade, especially the free isomeric L- or D-form is preferred.
India has abundant renewal agricultural resources that can be utilized as feed stocks for the production of lactic acid, which is a building block for the manufacture of polylactic acid (PLA).
Polylactic acid (PLA) is a biodegradable polymer derived from lactic acid. It is a highly versatile material and is made from 100% renewable resources like corn, sugar beet, wheat and other starch-rich products. Polylactic acid exhibits many properties that are equivalent to or better than many petroleum-based plastics, which makes it suitable for a variety of applications. Polylactic acid is a versatile polymer that has many potential uses, including many applications in the textile and medical industries as well as the packaging industry. Polylactic acid also has many potential uses in fibers and non-wovens. It is easily converted into a variety of fiber forms using conventional melt-spinning processes.
It is estimated that known global resources of oil will run dry in 80 years, natural gas in 70 years and coal in 700 years, but the economic impact of the depletion could hit much sooner; since prices will soar as resources are depleted. It is clear that researchers need to work toward replacing fossil fuel resources with renewable resources for many petroleum-based products. Headway is being made with a polymer called polylactic acid (PLA), an affordable, recyclable, innovative material made from renewable resources. Production of polylactic acid from renewable agricultural feed-stocks is an attempt to balance the resources and create plant based replacement for fossil-fuels. In addition, the excellent biodegradability of lactic acid based polymers and their environmental friendly nature of recyclability/compostability have further increased their potential and need for development.
The present invention is an attempt in this direction by providing an efficient process for producing polylactic acid from renewable agricultural feedstock like starchy materials, cellulosic materials like wood, cane bagasse, wheat straw, rice straw molasses, derived from cane or beet root.
Lactic acid can be manufactured by either chemical synthesis or renewable carbohydrate fermentation commercially. With increased public concern and government regulations on greenhouse gas emissions and environmental pollution, lactic acid produced by environmentally compatible fermentation bioprocesses, using renewable biomass resources, is preferable to chemical synthesis using fossil-fuels (coal, petroleum, or natural gas).
Polylactic acid is not a new material. It has been around for decades. In 1932, Wallace Carothers, a scientist from Dupont, produced a low molecular weight product by heating lactic acid under a vacuum. In 1954, after further refinements, Dupont patented Carothers' process.
Due to high costs, the focus since then has been mainly on the manufacture of medical grade sutures, implants and controlled drug release applications. The cost of production of the monomer has been a deterrent to widespread development of the polymer. Recently, there have been advances in fermentation of glucose, which turns the glucose into lactic acid. This has dramatically lowered the cost of producing lactic acid and significantly increased interest in the polymer.
Lactic acid, 2-hydroxy propionic acid or a-hydroxy propionic acid is manufactured/produced by synthetic and fermentation methods for use in food preservation, pharmaceuticals, leather tanning, and metal pickling as a starting material in specialized chemical processes. Two optically active isomeric (enantiomeric) forms of lactic acid are designated L(+) or S(+)-dextrorotary and D(−) or R(−) levorotary as shown below,

Chemical synthesis of lactic acid results in a racemic mixture wherein both enantiomers are present in equal proportion, whereas microbiological process produces predominantly one of the enantiomer.
L-lactic acid is necessary to produce biodegradable polymer. Production of L-lactic acid is normally achieved by selecting suitable microbial strain in the process of fermentation. It involves conversion of monosaccharides such as glucose, fructose, galactose or disaccharides such as sucrose or lactose into lactic acid. A few homofermentative strains producing only lactic acid as a product include Lactobacillus delbruekii, L. casei, L. acidophilus, L.bulgaricus. The former consumes sucrose, glucose or fructose but no lactose while the latter three species consume lactose and galactose in addition to the other sugars. The biological production of lactic acid is complicated due to inhibition caused by drop in pH due to lactic acid production and expensive downstream processing to produce lactic acid from dilute aqueous fermented broths. The conventional method of lactic acid production is the anaerobic fermentation by Lactobacillus sp. in batch reactors. In order to keep the fermentation process continuous, the acid produced is neutralized with alkali or removed from the fermentation system. The conventional processes employs calcium carbonate or sodium hydroxide to neutralize the acid produced. The corresponding lactate is then separated from the broth by various processes involving solvent extraction, electro-dialysis, or distillation or a combination of one or more process.
Production of polylactic acid requires high purity lactide obtained from predominantly L-lactic acid generated biochemically from edible renewable resource like carbohydrate feed-stocks. The conventional industrial processes involve removal of biomass from fermentation broth followed by acidification, purification, concentration and polymerization. The present invention describes an efficient process wherein L-lactic acid is predominantly produced from the fermentation of a cheap renewable agricultural feed-stock like molasses or cane bagasse hydrolysate which is separated, purified and concentrated concurrently to produce crude lactide which is polymerized to polylactic acid after further purification.
Fermentative production of lactic acid is the commonly used method for obtaining optically pure isomer required for production of polylactic acid. U.S. Pat. No. 6,475,759 (Nov. 5, 2002) granted to Carlson et al., provides a low pH lactic acid fermentation, which includes incubating acid tolerant homolactic bacteria in nutrient medium to produce a fermentation broth with high levels of free lactic acid. It also provides isolated acid-tolerant homolactic bacteria capable of producing high levels of free lactic acid. This patent relates specifically to bacteria capable of tolerating low pH and hence would be confined to the process, which would utilize that particular bacterial strain. The fermentation process employed herein is of batch type. Further the prior art uses calcium carbonate to neutralize the acid produced, which ultimately generates large amount of calcium sulfate (gypsum), which can pose a waste disposal issue, and calcium sulfate is considered to be an undesirable environmental concern.
Corn syrup, starch, corn-steep liquor, corn oil, milk-whey, sugar, beet and sugarcane juice are commonly used as feedstock for the process of fermentation. EP Pat No 0393818 A1 (Glessner, David A. et al;) provides a process for producing and purifying lactic acid, which comprises of growing a lactic acid producing microorganism on an inexpensive substrate containing carbohydrate, corn steep liquor and corn oil until most of the carbohydrate is converted to lactic acid. All these materials are of high value as food. In fact the cost of substrate is one of the current problems in the cost effective production of lactic acid by fermentation. The inventors of the present invention have used a cheap substrate, byproduct/waste product of sugar industry, molasses and cane bagasse hydrolysate as feed stock for lactic acid fermentation which does not have any value as food.
The above mentioned patent relates to a process for producing and purifying lactic acid by fermentation using robust strain of a lactate producing microorganism which produces lactate salt in high concentration from a low cost fermentation medium and a purification process which uses conventional electrodialysis to recover and concentrate the lactate from a whole broth containing cells and nitrogenous impurities; a water-splitting electrodialysis to convert the lactate obtained to lactic acid and base treatment with ion exchangers to remove charged impurities from the lactic acid. This process has limitations since expensive electrical energy, rather than other energy sources, is used to drive the process forward. Furthermore, polymeric membranes used in the process of electrodialysis are very sensitive to impurities and applying them to fermentation products of molasses would require costly purification operation.
EP Pat No 0790229A1 (Donald McOulgg, et al;) describes a process for the recovery of lactic acid from a medium by contacting it with a solid-phase free base polymer having tertiary amine or pyridine groups to absorb lactic acid. The lactic acid is later desorbed using a stronger acid or hot water. Loading of the resins by other undesirable ionic and acidic species in the fermentation broth, requirement of high regeneration efficiency and fouling of these resins by large organic molecules, pigments present in the broth are the major limitations of the process.
U.S. Pat No 6,569,989 (Ohara et al.), U.S. Pat. No. 6,326,458 (Gruber, et al.) and WO 93/00440 (Michael Cockrem et al;) relate to processes for producing lactide and polylactic acid from lactic acid obtained by fermentation, synthesizing lactate ester from lactic acid, distillation of lactate ester, polycondensation of the lactate ester in the presence of a catalyst to get a prepolymer of molecular weight in the range of 5000-15000, depolymerisation of prepolymer to get lactide and its ring opening polymerization whereby polylactic acid is obtained. Hydrolysis of the ester results in the lactic acid, if desired. This process involves the two steps of energy intensive distillation and in addition has the possibility of racemization.
U.S. Pat. No. 6,472,559 (Avraham et al;), EP Pat No 0804607 B1 (Abraham. M. Baniel, et al;) and U.S. Pat. No. 6,087,532 (Abraham. M. Baniel) describes a process, for the separation and recovery of lactic acid from fermentation broth using mixed solvents in the presence of CO2 and back extraction with water at elevated temperature of 80-240° C. The process generally involves preconcentration of the feed solution by water removal to the level of 40-70%, which is energy intensive. The process is associated with the limitations such as use of expensive solvents like high molecular weight trialkyl amines, difficulties in handling mixtures of solvents, their recovery and other associated problems.
U.S. Patent No. 5,369,122 (Steinmetzer) discloses a method for producing a humectant containing neutralized, concentrated L-2-pyrrolidone-5-carboxylic acid and lactic acid from sugar-free or partially desugarized residual molasses from sugar beet molasses. It involves preparation of sugar-free or partially desugarized residual molasses from sugar beet molasses by chromatographic separation of sugar beet molasses, using ion exclusion into fractions and separating the acids through steps of extraction and ion-exchange, and/or cation exchange and anion exchange, and/or cation exchange and elution with alkali metal hydroxide. The need for chromatographic separation of sugar beet molasses to obtain sugar-free or partially desugarized residual molasses for the recovery of acids, however, makes the process less attractive.
U.S. Pat. No. 5,310,865 (Katashi Enomoto et al;) discloses a process for making polyhydroxy carboxylic acid by conducting a dehydration condensation of a hydroxycarboxylic or an oligomer in a reaction mixture containing an organic solvent. The organic solvent is used to remove the water of condensation by azeotropic distillation. Preparation of very high molecular weight polymer above 1,00,000 required for many applications is one of the limitations of the process on account of difficulties associated with the removal of trace amounts of moisture.
The continuous process disclosed in U.S. Pat. No. 6,326,458 (Patrick Richard Gruber et al;) for the manufacture of lactide and lactide polymers involves preparation of lactide and lactide polymer from lactic acid or an ester of lactic acid in the presence of catalyst giving crude polylactic acid, prepolymer and a reaction by-product in the case of ester. The crude lactide obtained from prepolymer is purified by distillation again before being used for polymerization. The generation of by product in the case of lactic acid ester as starting material and the energy intensive step of distillation of crude lactide are constraints adding substantially to the cost of the process.
All the prior art methods for the production of lactic acid by fermentation either require relatively pure substrates for the growth of the bacteria and/or complex and toxic solvents for separation of lactic acid. Also, there are difficulties in recovering the solvents and high energy is utilized for separating such solvents by distillation. Thus, these processes are costly and time consuming. Some of the prior art provides a low pH lactic acid fermentation which includes incubating acid tolerant homolactic bacteria in nutrient medium to produce a fermentation broth with high levels of free lactic acid. Further the prior art uses calcium carbonate to neutralize the acid produced which ultimately generates large amount of calcium sulfate (gypsum), which can pose a waste disposal issue as calcium sulfate is considered to be an undesirable environmental concern.
Looking to the dire need of the hour, the scientists of the present have developed a novel process of producing polylactic acid, from fermentation of non-edible renewable agricultural feed stocks.
According to the present invention, polylactic acid is produced from fermentation of non-edible renewable agricultural feed stocks, the present process is found to be relatively cheaper as compared to conventional processes wherein starchy substrates are being used as a raw material.
In the present process, the pH adjustment is accompanied by using Ammonia, which does not form the salt precipitate (Gypsum) as in the conventional process. The present process is devoid of the formation of salt precipitate, thus the problem associated with the waste disposal issue has been resolved by the present invention.
Compared to the conventional process, the present invention provides a process wherein a single bulk solvent is used for the separation of lactic acid from fermentation broth at ambient temperature in comparison to a mixture of expensive solvents and pre concentration steps used in some of the conventional processes.
Further, the present invention provides a process wherein the regeneration/recovery of the solvent in the process are almost quantitative without any energy step of high temperature operation and/or distillation. The solvent thus regenerated is recycled for extraction without any further treatment.
In the present process the step of prepolymerization and lactide formation is combined in a single unit operation. Also the invention provides a process wherein the separation, concentration and purification is achieved concurrently using affinity driven processes at ambient temperature instead of energy driven processes employed in some of the conventional processes.
The present process developed by the inventors is efficient, cost effective and less cumbersome as compared to the conventional process.
It is still an object of the present invention to provide a process for producing polylactic acid from renewable feed stocks such as molasses as such without need for chromatographic separation by ion exclusion for better economics of the process.